The Fms-like tyrosine kinase 3 (FLT3) is a protein belonging to the class III of receptor type tyrosine kinases, and it has five immnunoglobulin-like motifs in the N-terminus extracellular domain, and two kinase domains at the C-terminus. Expression of FLT3 is observed on normal CD34-positive human bone marrow precursor cells and dendritic cell progenitors, and it plays an important role for proliferation, differentiation, and so forth of these cells (Non-patent document 1). Further, the ligand (FL) of FLT3 is expressed in bone marrow stromal cells and T cells, and is one of the cytokines that affect the cytogenesis of many kinds of hematogenous systems, and stimulate proliferation of stem cells, precursor cells, dendritic cells, and natural killer cells through interactions with other growth factors.
FLT3 is dimerized upon binding of FL, and activated by autophosphorylation. As a result, phosphorylation of PI3 as well as AKT and ERK in the RAS signal transduction pathway is induced. FLT3 plays an important role for proliferation and differentiation of hematopoietic cells.
In normal bone marrow, expression of FLT3 is limited to early precursor cells, but in blood carcinoma, FLT3 is expressed at a high concentration, or FLT3 causes a mutation and thereby contributes to proliferation and malignant alteration of carcinoma through activation of the aforementioned signal transduction pathway. The blood carcinoma include, for example, acute lymphocytic leukemia (ALL), acute myeloid leukemia (AML), acute promyelocytic leukemia (APL), chronic lymphocytic leukemia (CLL), chronic myeloid leukemia (CML), chronic neutrophilic leukemia (CNL), acute undifferentiated leukemia (AUL), anaplastic large cell lymphoma (ALCL), prolymphocytic leukemia (PML), juvenile myelomonocytic leukemia (JMML), adult T cell ALL, myelodysplastic syndrome (MDS), and myeloproliferative disorder (MPD).
As for AML among the various kinds of blood carcinoma, several existing therapies are effective to a certain extent, but relapse and resistance are frequently observed, and it is still such an intractable carcinoma as the five-year survival rate for that carcinoma is about 24% (in the United States) (Non-patent document 2). One of the causes of the relapse and resistance thereof is gene mutation of the AML cells, and especially, gene mutation of FLT3 is confirmed most frequently. It is known that the FLT3 gene mutation includes internal tandem duplication (ITD) mutation observed near the membrane (Non-patent document 3) and activation mutation of the tyrosine kinase moiety (Non-patent document 4), and FLT3 is constantly activated even in the absence of the ligand to accelerate proliferation of cancer cells.
It is reported that the ITD mutation, in particular, is observed in about 30% of AML patients, and vital prognosis of the patients having this mutation is poor (Non-patent document 5).
It is thought that suppression of both the activation of FLT3 and the activation thereof by gene mutation is important for the treatment of AML and improvement of prognosis, and development of FLT3 inhibitor is conducted.
For example, AC220 (Ambit) is a compound that selectively inhibits a type III tyrosine kinase (FLT3, c-KIT, FMS, PDGFR), and it is developed with targeting AML (Patent document 1).
Further, drugs showing superior sustainability are also being developed by covalently bonding such an inhibitory compound to a biological protein. For example, Afatinib (BIBW2992) is reported as an EGFR inhibitor having acrylic group in the molecule (Patent document 2).